feat: Add deployments query type

[twu]

Why?

Resolves #254

Users (and our squad) need to monitor GitHub deployments across different environments (e.g., production, staging) to track which version/hash has been rolled out. Our particular usecase is having Annotation queries to visualize when deployments have hit/made it into production. This is far from perfect but hopefully it's a good enough start.

What?

• Add deployments query type to support querying GitHub deployments via REST API
• Create GetAllDeployments function in pkg/github/deployments.go with support for filtering by SHA, ref, task, and environment
• Add GetDeploymentsInRange function to filter deployments by time range
• Implement DeploymentsWrapper.Frames() to convert deployments to Grafana DataFrames with fields: id, sha, ref, task, environment, description, creator, created_at, updated_at, url, statuses_url
• Add ListDeployments method to models.Client interface in pkg/models/client.go
• Create DeploymentsQuery and ListDeploymentsOptions models in pkg/models/query.go and pkg/models/deployments.go
• Add QueryEditorDeployments React component in src/views/QueryEditorDeployments.tsx with filter inputs for SHA, ref, task, and environment
• Integrate deployments query handler in pkg/github/deployments_handler.go and pkg/github/query_handler.go
• Add deployments query type to datasource handlers in pkg/github/datasource.go, pkg/plugin/datasource.go, and pkg/plugin/datasource_caching.go
• Add deployments constant to src/constants.ts and update query types in src/types/query.ts
• Register deployments query editor in src/views/QueryEditor.tsx
• Add test suite in pkg/github/deployments_test.go covering GetAllDeployments, filtering, time range queries, and DataFrame conversion
• Update test utilities in pkg/testutil/client.go to support deployments
• Add documentation for deployments query type in docs/sources/query/_index.md
• Add patch changeset entry for the new feature

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